Easiest way to diagnose misfires?

ejay

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I have a seemingly random P301 + P305 with very rough idle and diminished power. Blinking CEL. It's happened with my wife in the past but when I came to the rescue, it ran fine but kept the codes. (Don't remember if it was the same cylinders)

On the way home from taking the kids to play in the snow yesterday, shortbus decided to act up on me again out of the blue.

Any tips/tricks on the best way to test coils? Check individual injectors? Look for burnt valves? Generally track down the source of this issue? I have basic hand tools and power tools. Multi-meter, clamp meter, code scanner, but no noid light. (guess I could make one) Don't want to buy any special tools if I don't have to.

Jeep is an 04 4.0 and only has 125k. I've kept the fuel system clean. Cats will likely need to be replaced soon but they aren't plugged. (I pressure tested) O2 sensors are good as far as my tests go. Intake/exhaust manifold gasket and the exhaust manifold itself was changed out less than 10k ago. Haven't pulled the plugs yet. They probably only have 20k on them.
 

Jerry Bransford

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Exactly what brand and model # spark plugs are installed? Single-tipped platinum plugs like Autolite's AP-985 or Champion's 3412 show in some guide books as correct but they will cause misfires on later model 4.0 engines with the waste-spark ignition system like your '04 has.
 
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ejay

ejay

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Exactly what brand and model # spark plugs are installed? Single-tipped platinum plugs like Autolite's AP-985 or Champion's 3412 show in some guide books as correct but they will cause misfires on later model 4.0 engines with the waste-spark ignition system like your '04 has.
I honestly don't remember what I purchased when I did the job. In any case, they've been in there for some time and were in there for a while without issue. Could it be causing something as they deteriorate? I'll pull one when I get the chance and have a look.
 

TeeJohn

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That sounds a lot like both parts store crank sensors I’ve had to use. My OEM up and died at almost 150,000 miles. The first replacement lasted almost three days until it started that behavior. The next one started giving codes for misfire at speed, then finally acted like yours within a month. A new Mopar sensor has been trouble free for two months.
My trouble began on a long trip. Crank sensors are easy to replace but they don’t grow on trees beside the road. The first replacement did get me back into a town with several auto parts stores. The very old OEM unit stranded me on a Sunday in Lake City, CO so options were limited. I carry various spare sensors now. Neither mine nor auto parts code scanners listed a code specifically for the crank sensor, just misfire. The store selling the second replacement said to replace the cylinder head. Yeah. Just go find my sensor.
 
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ejay

ejay

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I wrote off the CPS as something not very likely but I'll add that to the list of things to test. Thanks.
 
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ejay

ejay

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I'll check the plugs tomorrow. Will need to get it up and running soon. Registration is due on the 12th and it requires smog this year. Just my luck......
 
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ejay

ejay

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Double platinum. Though the gap in the trouble code cylinder #1 has widened to .50. #5 was .25. It's as if I never set them in the first place. Lol I'm going to test the coil pack as soon as my wife gets home and can crank the engine for me while I watch.
20200107_163746.jpg

20200107_163754.jpg
 
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ejay

ejay

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Coils are fine. Spark on all cylinders. Going to put new plugs in tomorrow. After that, any ideas?
 
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ejay

ejay

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Thanks for the input. It has APP985 plugs but they could be swapped out due to condition. Well, I need to determine if I want to spend $50 on really nice spark plugs just to find out that it's the CPS. Then spend another $100 on a Mopar CPS. I did source one on Amazon, though, as I've read (and you mentioned) Mopar is the way to go. 56027865AB. Seems to be the correct part number. I've got until Saturday to fix, clear, and put on another 100 miles. Then smog and register. Don't think I'll make it in time before penalties.

Wouldn't a bad CPS not give you spark? Just like a a bad coil? I have spark on all cylinders. I'll test the resistance on the sensor leads before I commit to buy.
 
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ejay

ejay

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Sorry I missed that sale. I paid $8 a pop at Autozone today.

Update:
Coils tested fine last night. All cylinders sparked
Installed the Autolite XP985 plugs gaped at .040. Dielectric grease applied to the contacts. Antiseize in the threads.
Tested the CPS for resistance. None
Started her up.
Still misfiring on cylinder 1 and 5.
Cleared codes
Restarted and idled 'til warm. Got the RPM up a bit. Still misfiring but hasn't thrown the code yet.
Wife not home to watch the kids or I'd test drive. No doubt it would throw P301 & P305 again.
No other codes have been stored.

Any ideas?
 

Oldguy

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Start it in the dark with the hood open, no light at all. Look at all the plug wires for electrical arcing. If seen that wire is bad. Cap could also be bad
 
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ejay

ejay

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Have you checked the catalytic converters? One of my pre-cats self-gutted and clogged the 3rd cat recently
I pressure tested the exhaust system summer of last year. Not sure a clogged cat would give a misfire code as specific as this. Did you get misfire on all cylinders?
 

Steel City 06

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I pressure tested the exhaust system summer of last year. Not sure a clogged cat would give a misfire code as specific as this. Did you get misfire on all cylinders?
Yes, I only had a P0300 code (and limp mode). It's not as likely in your scenario, but it could cause issues